There is a continuing need for low-cost printing mechanisms for use with personal computers, facsimile machines, and the like, especially printing mechanisms that use so-called plain paper such as used in office photocopiers. There is also a need for small-size printing mechanisms that can be compatible with portable personal computers, especially those small enough to be incorporated in the computer case.
Serial impact printers have long filled the need for low-cost printing by using an array of needle-like print elements that are driven against an inked ribbon and paper to transfer ink from the ribbon to the paper surface. Serial printers move that array of print elements, called a printhead, across the page to print a band of text or graphic images. At the ending edge of the page the printhead is returned to the starting edge of the page while the page is moved forward the distance of one band. A next band of printing is then placed on the page adjacent to the previous band. Printed bands are successively printed until the page is filled or the printing task is complete. While being low cost, impact printers are limited in resolution and speed and produce more sound than is acceptable in many office environments.
Nonimpact printers, such as serial liquid ink jet printers, offer higher resolution and lower sound levels by using an array of individual droplet generators that eject liquid ink droplets toward a paper surface where they are deposited in the desired image pattern. The printhead and page motion are similar to that previously described for impact serial printers. A limitation of ink jet printers is that spreading of the liquid ink in the plain paper fibers can produce unacceptable images. A special coated paper is often required to produce acceptable image quality. Ink jet printers are also limited in printing speed by the droplet production rate unless large numbers of jets are used, which significantly increases the printer cost.
Nonimpact printers such as serial phase change ink jet printers use an array of individual droplet generators to eject droplets of molten wax-like material that solidify on impact with the papers surface before significant spreading can occur. This enables using plain paper, but often produces an objectional raised image similar to an embossed image.
Nonimpact line printers such as laser printers are able to meet the speed and plain paper requirements by printing the full width of the page at one time, rather than a portion of a band at a time as do the serial printers described earlier. Printing a full line at once increases the machine cost and size, preventing laser printers from meeting the low-cost or small-size requirements.
One printing technology that can meet the requirements of small size, plain paper, low sound, and low cost is disclosed in U.S. Pat. No. 5,036,341 wherein pigment particles are deposited directly on plain paper in the image pattern. The method of that patent brings an information carrier such as paper between a back electrode and a screen or lattice-shaped control electrode matrix of individual wires, all of which are connected to voltage sources. Voltage sources connected to the individual wires of the control electrode matrix at least partially open and close passages through the electrode matrix. Pigment particles are attracted from a particle carrier through the open passages and are deposited on the information carrier to form visible images. The paper is moved with respect to the electrode matrix to produce a line at a time.
The two-dimensional control electrode matrix of the above-mentioned patent is a lattice of individual wires arranged in rows and columns with one electronic drive circuit for each row or column wire. Operation of the control electrode matrix composed of individual wires can be sensitive to opening or closing of adjacent passages, resulting in undesired printing: a defect called cross-coupling.
U.S. Pat. No. 5,121,144 shows a control electrode matrix on a single insulating layer with one circular electrode surrounding each passage to eliminate the cross-coupling. The electrodes are arranged in rows and columns on a single insulating substrate with a single electronic drive circuit needed for each electrode. The use of one electronic driver per electrode on a single insulating layer is effective in eliminating cross-coupling, but increases the manufacturing costs by an undesirable amount because of the large number of electronic drive circuits required.